It is not uncommon to temporarily cease production from wellbore operations in order to perform auxiliary procedures, such as workover operations at different depths of a subterranean formation. Workover operations frequently use heavy brines and other fluids to maintain pressure control within the reservoir. Such fluids can leak-off into the production zone, causing damage which interferes with the efficient operation of the well.
Isolating the production zone protects it from damage. Specific blocking materials, such as solid blocking agents or temporary blocking gels isolate the production formation. Solid blocking agents, such as nylon or rubber balls, are injected into the fluid stream and seal the production formation by physically stopping up perforations in the formation. When the injection ends, the material is no longer held against the perforations and falls to the bottom of the well.
The use of temporary blocking gels successfully protects the production zone. Blocking gels formed by gelation of suitable polymers, such as appropriate polysaccharides produce a relatively impermeable barrier across the production formation. The barrier cordons off the production zone from the area undergoing the workover operations. These areas must remain separated until production is ready to resume.
Production resumes after removal of the blocking gel. The removal of the blocking gel is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids and pressure. This viscosity reduction or conversion is referred to as “breaking” and is often accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
Blocking gels have been used in low temperature zones for many years. However, certain problems have been associated with isolating high temperature subterranean zones, principally due to insufficient or unstable rheological properties of the crosslinked blocking gels.
Guar gum and guar gum derivatives, such as those containing hydroxypropyl guar (HPG) or carboxymethylhydroxypropyl guar (CMHPG), have been used to viscosify water and brine for the purpose of controlling fluid loss from oil wells during drilling, completions and workover operations. Such viscous polymer gels prevent fluid movement into subterranean formations by both high viscosity and filter cake deposits on the interface between the wellbore and the subterranean formation.
U.S. Pat. No. 5,547,026 discloses HPG and CMHPG blocking gels suitable for use in low to relatively high temperatures greater than about 250° F. to enable temporary isolation of the producing zone for an extended period of time. Blocking gels of greater thermal stability are desired.